This invention relates to video field artifact removal.
The NTSC video standard represents moving pictures using 30 individual frames per second. Each frame consists of two separate fields, which are sampled at different points in time. One field is displayed as the even scan-lines of a frame, and the other field is displayed as the odd scan-lines of the frame. If video is displayed at the intended rate, this allows a balance in the tradeoff between smooth motion and image resolution quality. However, if a single frame of video is viewed as a still, or if the format or frame-rate is changed, such as when video material is copied to film, these field artifacts can become visible and are undesirable.
FIG. 1 shows an example of 3 frames of video material in which an object is moving over a stationary background. The fields are apparent in regions where the object is moving, because each field represents a different point in time and therefore has a different appearance with respect to a moving object. FIG. 1D shows a detail from FIG. 1B where the motion of a white object has generated visible field artifacts in the form of stripes.
Field artifacts can typically be removed by blurring the image vertically or averaging the scan-lines of one field to replace the other. These techniques can successfully eliminate field artifacts, but the effective resolution of the entire image is also reduced.
The invention provides a process for adaptively removing field artifacts from digital video material. The process preserves the resolution quality of stationary areas of frames, while removing field artifacts where they exist.
Thus, in general, the invention features:
identifying, in an original frame of interlaced video fields, portions that represent motion in the frame,
generating a de-interlaced version of the frame, and
combining the original frame and the de-interlaced frame to form a resulting frame, the resulting frame including portions from the original frame that represent relatively less motion, and portions from the de-interlaced version that represent relatively more motion.
Implementations of the invention may include one or more of the following features: The portions of the fields and frames may be pixels. The portions that represent motion in the frame may be identified by comparing corresponding portions of at least two frames. The corresponding portions may be compared separately in different color channels. also including adjusting the results of the identifying step based on at least one of the following parameters: scale, threshold, or blur. Some portions of the de-interlaced version of the frame may be generated by a weighted averaging of portions of two fields of the frame. The de-interlaced frame may be generated in conjunction with a selected frame rate change.
Other advantages and features will become apparent from the following description and from the claims.